High speed serial links (i.e., HSSLs or high speed serial interconnects) are utilized in many embedded system designs, ranging from consumer and mobile devices to routers and switches that power the wired Internet backbone. The range of designs for which a developer may desire reliable and high quality signal integrity over interconnections range, for example, from various automobile rear-view camera systems, where the data rate may be less than 1 Gb/sec (gigabit(s) per second), to high-bandwidth Internet optical routers, where data rates may be 10 Gb/sec or greater.
As HSSLs approach higher speeds (e.g., 12 Gb/sec (SAS (Serial Attached SCSI (Small Computer System Interface) 4)) and 16 Gb/sec (PCI-E (Peripheral Component Interconnect Express) 4.0)), measuring signal quality has become increasingly difficult. With more sophisticated methods of equalization and emphasis, an increasing amount of signal loss may now be compensated for and an error free channel may be maintained. While these methods may lead to higher data rates using existing printed circuit board manufacturing technologies, externally measuring signal quality (e.g., with the aid of an oscilloscope) has specifically become more difficult.
To analyze and represent signal quality a high speed digital signal, a data eye (or data eye diagram) may be generated. The data eye enables key parameters of the electrical quality of the signal to be quickly visualized and determined. The data eye may be constructed from a digital waveform by “folding” the parts of a voltage signal waveform corresponding to each individual bit into a single graph with signal amplitude on one axis and time on another axis. By repeating this construction over many samples (e.g., clock cycles) of the waveform, the resultant graph or diagram represents the average statistics of the signal and resembles an eye. Because a received waveform may be degraded during transmission due to inter-symbol interference, jitter, crosstalk, noise and the like, system performance may be derived by analyzing the data eye corresponding to the received data eye. An “open” data eye may correspond to minimal signal distortion, while a “closed” data eye may imply distortion.
Voltage signals measured external to an electrical package may appear to have a closed eye and may undergo post-processing to determine the voltage signal that a receiver circuit may actually be obtaining. With post-processing in an oscilloscope, a voltage signal may externally appear of satisfactory quality, but may be deficient when received, for example because of the additional loss of the electrical package and the effects of the oscilloscope probing the voltage signal. Further, oscilloscopes may provide a segment of the voltage signal, rather than the complete voltage signal. Moreover, because some HSSLs may have interfaces with a large number of connector lanes, it may be an arduous task to utilize an oscilloscope to determine a data eye.